Method and apparatus for reducing distortion of digital data

ABSTRACT

A method of pre-distorting a signal of a satellite transmission link so as to offset later transmission of the signal during transmission across the satellite transmission link has a link including route Nyquist band pass filters in up and down links. The signal is passed through a cascade of identical pre-distorting stages each of which generates an approximation of the required pre-distortion. Each successive stage receives the approximation from the preceding stage so that errors in successive approximations converge towards zero.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the transmission of digital data, and inparticular to method and apparatus for reduction of distortion incurredby representations of digital data during passage through transmissionlinks.

2. Description of the Related Art

It is well known in the field of transmission of digital data, andparticularly for transmission of data concerning digital broadcastmaterial, for modulation techniques to use symbols, arranged as pointsin a particular constellation pattern to represent digital data. Typicaltechniques are those of phase Shift Keying (PSK) and QuadratureAmplitude Modulation (QAM). Two of the more common schemes of thesetechniques are those of Quadrature PSK (QPSK), e.g. digital satellitetransmission for “direct to home” applications, and 8PSK for e.g.satellite news gathering applications. There is a recognized desire toutilize higher order modulation schemes, such as 16 PSK and 16 QAM. Thiswill allow transmission at a higher bit rate, thus providing theopportunity to carry a greater number of channels within a pre-definedbandwidth of a particular transmission link.

Transmission of a modulated signal through transmission links such asterrestrial, satellite or cable links result in distortion of thesignal. This distortion is due, in part at least, to the non-lineareffects of passage of a signal through the transmission link. Distortionleads to a change in location of the constellation points of any givenmodulation scheme. An increase in the order of modulation results in adecrease in the distance between constellation points, and so leads to ahigher probability of distortion leading to errors occurring during thedemodulation of higher order modulation schemes.

Prior art methods used to reduce the effects of distortion by non-linearcomponent(s) within transmission links include use of at least partiallycompensating pre-correction. One approach is that of feed-forward, wherethe non-linear output of an amplifier is sampled, and compared with therequired signal prior to transmission, with the resultant error beingsubtracted from the amplifier output. This approach is only suitable forsystems in which the pre-distorter and amplifier are co-located. Thus,where this technique is used for satellite transmission links theopportunity for introduction or modification of the pre-distorter totake account of changes in amplifier characteristics is severelylimited.

Another technique is that of constellation pre-distortion, where theconstellation points generated by the modulator are pre-distorted suchthat at the amplifier output the constellation points are located intheir correct relative positions. This method is suitable only fortransmission links that are memory-less. This precludes the use of thisapproach where pulse shaping takes place before non-linear amplificationof the signal. Therefore, it is not suitable for those transmissionlinks that include bandpass filtering of the signal.

Signal pre-distortion performed at the radio (RF), intermediate (IF) orbase band frequencies is often carried out by application of an inversefunction of the distortion to the signal as disclosed in WO-A-95132561and U.S. Pat. No. 4,992,754. This type of pre-correction generates outof band components, which are then carried through to the amplifierinput. Where the amplifier has an input filter, as is common foramplifiers used in satellite transmission links, then these componentsmay be removed from the signal which becomes the input to the amplifier.Thus, the distortion imposed by the amplifier will not be accuratelycorrected as the amplifier input signal is not the entire transmittedsignal. This means that this form of pre-correction is not effective forcorrection of amplifiers contained within satellite transponders wherethe bandwidth of the incoming signal is high in relation to thebandwidth of the transponder. Additionally, for digital transmissionusing higher order modulation schemes, this type of pre-correctionrequires very high clocking rates in order to generate the wide-bandpre-distortion components.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome at least some ofthe problems associated with the prior art.

According to this invention there is provided a method of pre-distortinga signal of a satellite transmission link, said signal being modulatedto carry symbols representative of digital data, so as to offset laterdistortion of the signal during transmission across the satellitetransmission link, said link having root Nyquist bandpass filters inrespective up and down links, the method including passing the signalthrough a cascade of identical pre-distorting states, each of whichgenerates an approximation of the required pre-distortion, eachsuccessive stage receiving the approximation from the preceding stage sothat errors in successive approximations converge toward zero withincrease in the number of stages.

According to a second aspect of this invention there is provided asatellite transmission link including root Nyquist bandpass filters inrespective up and down links and apparatus for pre-distortion of asignal, modulated to carry symbols representing digital data, so as tooffset later distortion of the signal during transmission across saidlink, the apparatus comprising a cascade of identical pre-distortingstages, each said stage having means for generating an approximation ofthe required pre-distortion, and each successive stage being connectedto receive the approximation from the preceding stage so that the errorsin successive approximations converge toward zero with increase in thenumber of stages.

The method and apparatus of this invention allows input of a complexsignal at a rate as low a one sample per symbol to the pre-distorter,and generating at its output a complex signal which may be at the samerate. This means that implementation of the hardware is practical forsystems operating at higher symbol rates.

The method and apparatus of the invention are particularly suited topre-distortion of a modulated signal which is subsequently transmittedthrough a satellite transmission link as it provides a ground basedmeans of applying pre-distortion of the amplifier located on thesatellite.

It is common for transmission links to include band pass filters betweenthe means of modulating the signal and the amplifier. As is describedabove, such filters are known to remove at least substantial portions ofany out of band components contained within a signal. Additionally, thisinvention provides accurate pre-distortion for transmission links havingone of more band pass filter regardless of the location of suchfilter(s). Satellite transmission links commonly employ ground andsatellite-based band pass filters.

The method and apparatus of this invention allows accuratepre-distortion for a transmission link carrying any constellationpattern and having any non-linear amplifier, irrespective of whether thelink is memory less or not.

This allows pre-distortion to be applied to a signal for subsequenttransmission through a transmission link having a band pass filter ateach of the transmitter and receiver ends of the link. Additionally, bytaking past and future symbols of the signal into account, not only canthe static position of the constellation points to be pre-distortedaccurately to take account of the effects of passage through thenon-linearity of the link, but also the effects of inter-symbolinterference (i.e. smearing) are substantially reduced.

The invention will now be described by way of example only and withreference to the following figures:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a satellite transmission systemincorporating a satellite transmission link.

FIG. 2 is a schematic diagram of a pre-distorter of the presentinvention.

FIG. 3 is a schematic diagram of the distorting function FM1 of FIG. 2.

FIG. 4 is a representation of an ideal 16 QAM constellation prior totransmission through a transmission link.

FIG. 5 is a representation of the output from a receiving Nyquist filtercorresponding to the transmission link input of FIG. 4.

FIGS. 6 a to 6 d show computer simulations of outputs from a receivingNyquist filter when different numbers of successive approximation stagesare used.

FIG. 7 is a graphical comparison of Bit Error Rates of differingpre-distortion circumstances.

FIGS. 8 a and 8 b show the output of a receiver Nyquist filter for 32QAM using a pre-distorter of the invention, and the correspondingconstellation at the input to a transmitter Nyquist filter respectively.

FIG. 9 shows a computer simulation of the output spectrum of thepre-distorter in comparing it with the spectrum without the use ofpre-distortion.

FIG. 10 is a schematic diagram of a feedback control loop.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is shown a satellite transmission link 1, having a rootNyquist band-pass filter 2, IQ modulator 3 and up-converter 4 prior to atransmitter 5. The transmitter provides an uplink to a satellite 6,which in turn provides a downlink to a number of receivers, one of whichis shown as receiver 7. The receiver end of the satellite transmissionlink 1 can be seen as a reverse of the transmission end, with thereceiver 7 connected successively through a down-converter 8, an IQdemodulator 9 and a root Nyquist band-pass filter 10.

During operation of the transmission link 1, an input base bandfrequency signal, being one which has been modulated by a particulartechnique and scheme such as 16 QAM and having I and Q complexcomponents, is filtered by the root Nyquist band-pass filter 2. It isusual to use Nyquist filtering within transmission links in order toconstrain the bandwidth of the transmitted signal. Conveniently Nyquistfiltering of the signal is conducted by root Nyquist filters placed ateach of the transmitter and receiver ends of the transmission link.Nyquist and root Nyquist filters impose a linear distortion upon themodulated signal.

Each of the I and Q components of the root Nyquist filtered signal areprovided as inputs to the IQ modulator 3 such that the input toup-converter 4 is a modulated carrier which represents the digital dataof the chosen modulation scheme. The up-converter changes the frequencyof the input signal to a higher frequency than base band. The higherfrequency signal is transmitted to satellite 6, where it is received bythe satellite's transponder (not shown), amplified by a travelling wavetube amplifier (TWT) and retransmitted from the transponder to aplurality of receivers 7. The transponder is often constructed such asto ensure that the transponder input is passed through band-pass filtersbefore and after being amplified by the TWT. Band-pass filters includethe characteristics of not allowing out of band components to passthrough the filter.

On receipt of the transponder output by receiver 7 the signal isprocessed by each of down-converter 8, IQ demodulator 9 and root Nyquistfilter 10 to provide an output from the transmission link 1 in the formof a base band frequency signal having I and Q components. This outputis then subsequently demodulated to obtain the digital data transmittedby the symbols within the modulation scheme.

In addition to the satellite transmission link 1, FIG. 1 also shows apre-distorter 12 located prior to the transmission link 1. Thepre-distorter is adapted to operate in accordance with the invention ofthis application, and arranged to apply pre-distortion to an incomingsignal to compensate for the distortion subsequently applied to thatsignal during its passage through the satellite transmission link 1.

FIG. 2 is a block diagram of the pre-distorter 12 in greater detail. Itis assumed that all signals (input to, output from and within thepre-distorter) are complex signals. However, it will be understood bythose skilled in the art that in a practical implementation these signalcould be in the form of either Cartesian or polar representation. Theinput signal 19 to the pre-distorter 12 is fed from junction 20 to aforward model of a distorting function 21 representative of thedistortion of the satellite transmission link to supply symbolsrepresentative of digital data for time=t(1). The output 22 of thisforward model is input along with the input signal 19 (which is arrangedto be supply symbols representative of digital data for time=t(1) by useof unshown delay apparatus) at summing node 23. The output of summingnode 23 is scaled by a value A by a multiplier 24. The output ofmultiplier 24 is combined with input signal 19 (also arranged to supplysymbols representative of digital data for time=t(1) by use of unshowndelay apparatus) to summing node 25 to provide an output 26 from thefirst stage of approximation. This output 26 is determined by equation(1) given below.Output 26=(input 19)*A−(output 22)*A+(input 19) Output 26=[(input19)−(output 22)]*A+(input 19)  Equation 1[(input 19)−(output 22)] is the error which exists between output 22 andinput 19, and thereforeOutput 26=(input 19)−(error*A)  Equation 2

It will be understood that Output 26 concerns symbols representative ofdigital data for time=t(1).

Output 26 acts as the input forward model 27 of the identical secondstage of successive approximations, and is the same as input 19 to thefirst stage, but modified by the error scaled by a factor A. It can beseen that the pre-distorter 12 generates an error for symbolsrepresentative of digital data for time=t(1) in relation to the outputof forward model 21 of the first stage of the successive approximationsbut the correction to account for pre-distortion is applied in relationto the input to forward model 27 of the next stage. During passage ofsymbols representative of digital data for time=t(1) through the secondstage of successive approximation, the first stage of successiveapproximation is supplied at junction 20 with symbols representative ofdigital data for time=t(1+n) where n represents the pipeline delay.

It can be shown that with a suitable choice of A the use of successiveapproximations such as given in the pre-distorter 12 of FIG. 2 then theoutput of the pre-distorted will converge towards zero error as morestages of successive approximation are added. In practice A is chosen toachieve the highest convergence rate for a given forward modeldistorting function. For a transmission link such as that of FIG. 1,then it has been found that six stages of successive approximationstrikes a reasonable balance between convergence towards zero error andhardware implementations of the pre-distorter. Optionally, pre-distorter12 may additionally include an initial approximator 28. This initialapproximator is arranged to operate function IM1, which is a functionarranged to provide an output which is approximately the inverse offorward model distorting function FM1 as implemented by forward modeldistorting functions 21, 27 and their equivalents in further successivestages. Where initial approximator 28 is included within a pre-distorter12, then output 29 is provided as an input to distorting function 21 andinstead of input 19 fed from junction 20. This is arranged to ensure(through use of appropriate delays to operate for symbols representativeof digital data for time=t(1) as described above.

Thus equation (2) becomesOutput 26=(Output 29)−error*A  Equation 3

For pre-distortion of an amplifier such as a TWT, the initialapproximator 28 may be a function which places the constellation pointsin the correct place for pre-distortion but which does not dynamicallychange their position from symbol to symbol. This is known as a staticpre-distortion. Using a static pre-distortion for a rough approximationsubstantially reduces the number of successive approximation stagesrequired. Typically one static correction stage plus three dynamicstages (implemented by passage through one successive approximationstage) is sufficient to attain the desired pre-distortion of a satellitetransmission link such as that of FIG. 1.

The distorting function 21 for the transmission link 1 of FIG. 1 may beof the form shown in FIG. 3. This consists of a forward TWT model 30,with root Nyquist filters 31 and 32 placed before and after the model.It is the presence of the root Nyquist filters which enables thepre-distorter 12 to correct for the signal transitions from oneconstellation point to another. The practical implementation of the rootNyquist filters enables the summation of scaled sample values. Thesample values include both past and future samples. This is themechanism that enables correction for the effects of distortion uponsymbols which rely upon past and future symbols for their laterinterpretation during demodulation, i.e. the dynamic distortion. Whenused in combination with successive approximation this allows for asubstantial reduction in the effects of inter-symbol interference.

The method and apparatus of this invention can be adapted in the mannerexemplified in this embodiment so as not to generate out of bandcomponents. Instead, the non-linear correction components are foldedback into the bandwidth of the signal, and thus provide accuratepre-distortion for transmission links having one or more band passfilters.

If it is required that the pre-distorter shall correct additionally forthe uplink High Power Amplifier (HPA), then a forward HPA model can alsobe included in in the forward model distorting function. Alternatively,a completely separate pre-distorter could be used.

As an example of the operation of a pre-distorter, some simulationresults are given in FIGS. 4 to 9. In this example A=0.875=⅞. The use ofsimple fractions with a binary denominator reduces hardware complexity.A Nyquist link with 35% roll-off factors is included.

FIG. 4 shows an ideal 16QAM constellation to be transmitted with thecorner points placed at TWT saturation and FIG. 5 shows thecorresponding output from a typical TWT. The constellation points aredisplaced and additionally they are smudged due to the transitionaffects caused by the transmit Nyquist filter/TWT combination.

FIGS. 6 a to 6 d show the result corresponding to the outputs ofsuccessive pre-distorter stages. It can be seen, when comparing theresults of FIG. 6 a with those of FIG. 5 (no correction by means of apre-distorter), that passage through just one stage of successiveapproximation leads to an improvement in the dynamic distortion imposedby a satellite link such as that of FIG. 1. The result can be improvedby adding further pre-distorter successive approximation stages, as canbe seen by comparison of the results of FIGS. 6 b to 6 d.

Use of greater numbers of successive approximation stages leads to allpoints, except for the four corner points, becoming nearer to beingsubstantially ideal following passage through each additional successiveapproximation stage. The corner points, which are at TWT saturation,exhibit a comet tail effect. This is due to the fact that, at thesepoints, the gain of the TWT is zero and perfect convergence is notpossible. Nevertheless the link Bit Error Rate (BER) approaches that ofthe linear link.

FIG. 7 shows a comparison of a satellite transmission link BER forvarious uncoded modulations wherein static and dynamic pre-distortionshould be noted.

Line 70 represents the BER for a signal modulated by 8PSK andtransmitted through a transmission link having no non-linear distortion.Thus, this is also the ideal signal output BER of a transmission linkfollowing passage through a pre-distorter. Line 71 represents the sameideal for the different modulation scheme of 16QAM. Line 72 demonstratesthe BER of a signal having been modulated by 8PSK and beingpre-distorted by the pre-distorted by the prior art technique of staticpre-distortion. Graph lines 73 and 74 represent the BERs of the 16QAMmodulated signals having static and dynamic pre-distortion, and staticonly pre-distortion respectively. It should be noted that a 16QAMmodulated signal subjected to both static and dynamic pre-distortiondemonstrates better performance than a signal modulated by 8PSK andpre-distorted by the prior art techniques of static only pre-distortion.As explained earlier, higher order modulation schemes are inherentlymore susceptible to increased BER due to the decreasing distance betweenconstellation points. FIG. 7 thus demonstrates the significant increasein performance associated with use of the pre-distortion method andapparatus of this application.

32 QAM is particularly suited to pre-distortion in accordance with thisinvention because the corner points are missing and all the remainingpoints pre-correct well. FIG. 8 a shows the output from a satellitetransmission link TWT with pre-distortion using 6 stages of dynamicpre-distortion only. A=0.875 and 35% roll off factor. The corner pointsthat are missing would be at the saturation point of the TWT if theywere present. FIG. 8 b shows the pre-distorter output, which is a 32QAMconstellation to be transmitted through a satellite transmission link.FIG. 9 shows the output spectrum from the uplink for the case with andwithout pre-distortion. It is clear that in both cases the spectrumfollows the shape of the transmit Nyquist filter and the only differenceis that the pre-distorted signal is about 34 dB lower in power.

In the above description is has been assumed that the characteristics ofthe TWT are known and these parameters are programmed into thepre-distortion hardware.

FIG. 10 illustrates a block diagram of a feedback control loop, whichincludes a satellite of satellite transmission link 1, for modificationof the pre-distortion parameters of the forward model distortingfunction FM1 of FIG. 3.

The pre-distorter 100 receives its control parameters frommicroprocessor 101. The required non-linear characteristics aregenerated by the microprocessor and are down-loaded into RAM in thepre-distorter. Two modes are possible. In the non-feedback mode, it isassumed that the parameters for a particular satellite are known andthese are stored in memory 102. The accuracy of pre-distortion will belimited be the accuracy of these parameters.

In the feedback mode, the satellite output is received by receiver 103and the constellation analyser 104 generates error signals, whichcorrespond to the displacement of the constellation points. For 16QAMfor example the constellation points fall on three circles, so amagnitude and phase error estimate may be made for each of the threecircles. The error signals are processed by a software algorithmimplemented in the microprocessor to generate updates of the controlparameters passed to the pre-distorter.

It will be realised that the time delay of the return path to thesatellite is approximately 0.25 seconds, and so the update time of thecontrol parameters must be equal or longer than this delay. However, inpractice the TWT parameters will only be slowly varying.

In the feedback mode described above, no training sequence is required.However, if the parameters of the satellite TWT are not known, then theconstellation from the TWT may be so distorted that the trackingfeedback loop cannot pull into lock.

One solution for this is to add a training sequence into thetransmission. An alternative method is to provide a separate trainingsignal. When the up-link is first operational into the satellite, a setup mode in the modulator generates a training signal, which may not beof the same modulation type as the intended transmission. Themicro-processor runs an algorithm to calculate the required correctionparameters based on the signal from receiver 103. Subsequent to thisprocedure, transmission of the intended transmission may start and thefeedback mode accurately maintains the control parameters. In the caseof intermittent loss of the received signal, the corrector continues tocorrect accurately because the most recent control parameters are storedin the pre-distorter RAM.

1. A method of pre-distorting a signal of a satellite transmission link,said signal being modulated to carry symbols representative of digitaldata, so as to offset later distortion of the signal during transmissionacross the satellite transmission link, said link having root Nyquistbandpass filters in respective up and down links, the method includingpassing the signal through a cascade of identical pre-distorting stages,each of which generates an approximation of the required pre-distortion,each successive stage receiving the approximation from the precedingstage in an iterative fashion so that errors in successiveapproximations converge toward zero with increase in the number ofstages.
 2. The method of claim 1, wherein the transmission link has aparticular bandwidth and wherein the signal is passed through a cascadeof pre-distorting stages, each of which generates an approximation ofthe required pre-distortion within the said bandwidth.
 3. The method ofclaim 1, wherein said signal is applied to a forward modelrepresentative of the distortion of the satellite transmission link, anoutput of the forward model is added with said signal to provide anerror signal, said error signal is amplified and further summed withsaid signal to provide an input to a next succeeding stage.
 4. Themethod of claim 1, wherein said signal is passed through an initialapproximator prior to passage through successive approximation stages.5. The method of claim 4, wherein the initial approximator comprises astatic pre-distortion approximation function.
 6. The method of claim 1,wherein the signal is modulated in accordance with 16 QAM.
 7. The methodof claim 1, wherein the signal is modulated in accordance with 32 QAM.8. The method of claim 1, wherein the signal is modulated in accordancewith 16 PSK.
 9. A satellite transmission link including root Nyquistbandpass filters in respective up and down links and apparatus forpre-distortion of a signal, modulated to carry symbols representingdigital data, so as to offset later distortion of the signal duringtransmission across said links, the apparatus comprising a cascade ofidentical pre-distorting stages, each said stage having means togenerate an approximation of the required pre-distortion, and eachsuccessive stage being connected to receive the approximation from thepreceding stage so that the errors in successive approximations convergetoward zero with increase in the number of stages.
 10. A link accordingto claim 9, wherein the transmission link has a particular bandwidth,and wherein each pre-distorting stage is arranged to generate anapproximation within the said bandwidth.
 11. A link as claimed in claim9, wherein each pre-distorting stage includes a forward modelrepresentative of the distortion of the satellite transmission linkarranged to receive said signal, a summer to add an output of saidforward model with said signal to provide an error signal, an amplifierto amplify said error signal, and an output of said amplifier beingapplied to a further summer to add an output of said amplifier with saidsignal, wherein an output of said further summer may be applied as inputto a forward model of a next succeeding stage.
 12. A link as claimed inclaim 9, wherein an initial approximator is connected to provide inputto a first of said pre-distorting stages.
 13. A link as claimed in claim12, wherein the initial approximator comprises a static pre-distortionapproximation model.
 14. A link as claimed in claim 9, wherein thesignal is modulated in accordance with 16 QAM.
 15. A link as claimed inclaim 9, wherein the signal is modulated in accordance with 32 QAM. 16.A link as claimed in claim 9, wherein the signal is modulated inaccordance with 16 PSK.
 17. A link as claimed in claim 9, wherein sixsuccessive approximation stages are provided.
 18. A link as claimed inclaim 13, wherein one static pre-distortion and three successiveapproximation stages are provided.
 19. A link as claimed in claim 9,wherein a feedback control loop is provided from the satellitetransmission link to the cascade of pre-distorting stages.
 20. A link asclaimed in claim 19, wherein the feedback control loop includes areceiver to receive signals transmitted by a satellite, a constellationanalyzer to generate error signals produced by the receiver, in whicherror signals are representative of a displacement of constellationpoints in the signal, the constellation analyzer to provide output to aprocessor to produce non-linear characteristics as an input to saidapparatus.